i think that most kids regress in some area of their life when a new sibling comes. but it is not necessarily the potty. DD1 was 26 months when DS was born. she was not 100% a graduate, though she had been an at-home grad for several months we still had some misses out and about. DS was maybe 3 or 4 weeks old at the most when DD1 totally stopped having those. she wasn't a night-time grad for another maybe 2 or 3 months after that, but i never EC'd her at night, she just started being always dry in the morning around 28 or 29 months. *but* she nursed almost as much as the baby, and suddenly wanted to be worn again. she also had some sleep disruption, and woke more at night for a brief period of time, though that might've been just adjusting to an additional person in the family bed, and having me feeding and changing and such in the room at night while she was sleeping.
DS was 33 months old when DD2 was born. for some reason i can't remember exactly when he graduated.... it was somewhere around 28 months, though he had been mostly a grad since about 20 months, that was when he was totally 100% about telling me. so anyway, it hadn't been too long when DD2 was born, but there was no regression at all. he did nurse more too, though not nearly like how DD1 did (of course he was a little older), and i think that was less of a regression than just being reminded about nursing because he saw it going on constantly. he did ask to be worn for a while after she was born, and was a little more whiney and clingy.
so anyway, i don't think that new baby always = potty regression, but you should probably plan on some kind of regression on some level regarding something if you're planning a new sibling.